1. Field of the Invention
The present invention relates to a method for producing a resin-coated carrier used in electrophotography in which a latent image formed on an image bearing member is developed into a visible image, a two-component developer containing the resin-coated carrier, a developing device using the two-component developer, an image forming apparatus, and an image forming method.
2. Description of the Related Art
Office automation (abbreviated as “OA”) equipments have been remarkably developed in these days and in line with such development, there has been a wide spread copiers, printers, facsimile machines, and the like machines which form images through electrophotography.
For example, an image is formed by way of a charging step, an exposing step, a developing step, a transferring step, a fixing step, and a cleaning step in an image forming apparatus which employs electrophotography. At the charging step, a surface of a photoreceptor serving as an image bearing member is evenly charged in a dark place. At the exposing step, the charged photoreceptor receives signal light derived from a document image, resulting in removal of charges on the exposed part of the photoreceptor whose surface thus bears an electrostatic image (an electrostatic latent image). At the developing step, an electrostatic-image-developing toner (hereinafter simply referred to as “toner” unless otherwise mentioned) is supplied to the electrostatic image on the surface of the photoreceptor, thereby forming a toner image (a visualized image). At the transferring step, the toner image on the surface of the photoreceptor is transferred onto the recording medium by providing the recording medium with charges of which polarity is opposite to that of charges of the toner. At the fixing step, the toner image is fixed on the recording medium by heat, pressure, or the like. At the cleaning step, the toner is collected which has not been transferred onto the recording medium and thus remains on the surface of the photoreceptor. Through the above steps, a desired image is formed by the image forming apparatus employing electrophotography.
A usable developer for developing an electrostatic image in the image forming apparatus employing electrophotography includes a one-component developer containing only a toner and a two-component developer containing toner and carrier. The two-component developer is provided with functions of stirring, conveying, and charging toner by the carrier. Accordingly, since toner in two-component developer does not need to have functions of carrier, the two-component developer has characteristics that the controllability is improved due to such separation of the functions, and a high-quality image is easily obtained, compared with one-component developer containing toner solely. Therefore, a lot of development and research have been conducted with respect to toner suitable for use in combination with carrier.
A carrier has two fundamental functions: the function of stably charging a toner to a desired charge level, and the function of conveying a toner to a photoreceptor. Furthermore, a carrier is stirred in a developing tank, and borne onto a magnet roller, on which the carrier forms a magnetic brush. Subsequently, the carrier passes through a regulating blade, and then returns to the inside of the developing tank. This allows the carrier to be reused. In continuing use of the carrier, the carrier is required to stably realize the fundamental functions, particularly the function of stably charge a toner. However, the carrier generally has large density and large stirring torque. Therefore, much driving power is required to stir the carrier in a developing tank.
In recent years, in view of environment, improvement in a carrier relating to low power consumption of an image forming apparatus is developed, and many investigations to decrease a density of the carrier are conducted to achieve low power consumption by reducing stirring torque of a developing tank. Furthermore, a carrier having low density tends to be investigated in the standpoint of long life of a carrier. To realize low density of a carrier, it is important to decrease density of a core material itself of the carrier.
For the purpose of solving the above problems, Japanese Unexamined Patent Publications JP-A 2-220068 (1990), JP-A 3-192268 (1991) and JP-A 4-86749 (1992) disclose a magnetic powder-dispersed resin carrier that tried to decrease its density by using a comparatively small ferromagnetic substance and incorporating the substance into a thermal crosslinking resin.
JP-A 2006-337579 and JP-A 2007-57943 disclose a carrier in which pores of a carrier core material having pores therein (hereinafter referred to as a “porous type”) are filled with a resin to decrease a density, and the surface of the carrier core material is coated with a silicone resin.
However, the magnetic powder-dispersed resin carriers disclosed in JP-A 2-220068, JP-A 3-192268 and JP-A 4-86749 are that because a magnetic substance used is a ferromagnetic substance, residual magnetization is large, adhesion by magnetic force is generated between carrier particles, and furthermore, a carrier is liable to remain on a magnet roller in the inside of a developing tank. Therefore, those carriers give rise to the problem in stirring property.
The amount of a resin used to coat a carrier core material is generally about 2 parts by weight to the carrier core material. However, the carriers disclosed in JP-A 2006-337579 and JP-A 2007-57943 require the amount at least 10 parts by weight, and this is not realistic from the production standpoint. Specifically, costs required in the production of a carrier are increased with increasing the amount of a resin used. Furthermore, because the amount of a resin used is large, it is difficult to control a thickness of a resin coating film which coats the surface of a carrier core material filled with a resin. Where a resin is added in an amount such that pores of the carrier core material are sufficiently impregnated therewith, carrier particles are liable to be adhered each other, and a uniform resin coating film cannot be formed. The carriers disclosed in JP-A 2006-337579 and JP-A 2007-57943 are that a resin coating film is formed by a wet process. Thus, the carriers contain an organic solvent, and therefore, a stable resin coating film cannot be formed.